Shell

Variables

All active variables can be seen by running env.

$HOME - Expands to the path of my home folder.
$PS1 - Variable that represents my command prompt line.
$PATH - Special environment variable that contains the command path (list of system directories that the shell searches when trying to locate a command).

Notes

Scripts are run in subshells, and nothing is shared "upwards". That's the difference between running a script and sourcing one. A sourced (imported) script is run in your own script's namespace.
In shell everything is a string.
Children never touch parent environment. It can only if it runs as part of the current process (source, function, alias).
Pipes are used to connect one process's output with another process’s input.
/etc/paths.d define paths to add to $PATH globally to all users.

Links

Explain Shell
Introduction to POSIX Shell
Yoshua's notes
Shell Auto-completion Systems
Shell and Scripting (2019)
ShellCheck - Finds bugs in your shell scripts.
Rash - The Reckless Racket Shell.
Eternal Terminal - Remote shell that automatically reconnects without interrupting the session.